In recent years, handheld terminal devices such as laptop personal computers, cellular phones, and PDAs (Personal Digital Assistants) are being remarkably widespread. As the secondary batteries as the power source in these handheld terminal devices, e.g., lithium ion secondary batteries are often used. The handheld terminal devices are required to have a comfortable portability, and such devices are rapidly becoming more compact, thin and lightweight with better performance. As a result, the handheld terminal devices are now being used in a wide variety of situations. Like the demand on the handheld terminal devices, there also is a demand on the batteries to be smaller, thinner and lighter with better performance.
In a lithium ion secondary battery, upon charging, lithium elutes as a lithium ion from a positive electrode active material in a positive electrode to an electrolyte solution in an organic separator, and then enters into a negative electrode active material in a negative electrode. Upon discharging, the lithium ion that has been entered into the negative electrode active material in the negative electrode is discharged to the electrolyte solution, and returns to the positive electrode active material of the positive electrode. In this manner, act of charging/discharging is performed.
As the organic separator for use in a lithium ion secondary battery, e.g., a microporous membrane formed of a polyolefin resin is usually used. When inner-battery temperature rises to around 130° C., the organic separator melts and occludes the micropores. The organic separator thus has a shut-down function that inhibits migration of lithium ions and cuts off the electric current. In this manner, the organic separator plays a role of keeping safety of the lithium ion secondary battery. However, when the battery temperature exceeds, e.g., 150° C. due to instantaneously generated heat, the organic separator rapidly shrinks, and the positive electrode and the negative electrode directly contact to each other, to cause enlargement of short-circuited area. In this case, the battery temperature can rise to several hundred degrees Celsius, to be in a state of abnormal overheat.
In order to address this problem, there has been made studies on, e.g., a non-aqueous separator having an organic separator such as a polyethylene microporous membrane, and a heat-durable porous membrane layer (this may be referred to hereinbelow as “heat-durable layer”) laminated on the surface of the organic separator. The porous membrane layer is a membrane having therein a large number of connected microporous structures. The porous membrane contains non-conductive particles, and a polymer binder for effecting binding of the non-conductive particles to each other, and binding of the non-conductive particles with the organic separator or a current collector (this polymer binder may be referred to hereinbelow as “binder”).
The porous membrane layer may also be used in a form of being laminated on the electrode, or used as the organic separator itself.
Patent Literature 1 proposes a non-aqueous secondary battery separator including a polyolefin microporous membrane, and a heat-durable porous layer that is provided on one or both surfaces of the polyolefin microporous membrane and contains a heat-durable resin, as well as a non-aqueous secondary battery separator including a polyolefin microporous membrane, and an adhesive porous layer that is provided on one or both surfaces of the polyolefin microporous membrane and contains a vinylidene fluoride resin.
According to Patent Literature 1, the heat-durable porous layer preserves the polyolefin microporous membrane even at a temperature that is equal to or higher than the shutdown temperature, and thus reduces possibility of meltdown, to thereby ensure safety in a high temperature occasion. Patent Literature 1 also discloses that the adhesive porous layer improves adhesion with the non-aqueous secondary battery separator, whereby, in addition to the mechanical strength, shutdown property and anti-liquid depletion effect attributed to the polyolefin microporous membrane, the adhesive porous layer exerts excellent ion permeability and electrolyte solution preservability.